2. The Field of the Invention
This invention relates to apparatus used to determine blood pressure of a subject. More particularly, the present invention relates to methods and apparatus which are used to noninvasively and continually determine a subject's blood pressure.
3. The Prior Art
Out of all of a patient's physiological signs which can be monitored by medical practitioners, the physiological parameter which practitioners would prefer to have monitored for critically ill patients, if only one physiological sign could be monitored, would be the patient's blood pressure. Cardiologists, anesthesiologists, internists, and other practitioners dealing in many areas of medicine consider blood pressure measurements to be extremely valuable. Investigators in the basic life sciences often measure blood pressure under a wide variety of circumstances.
There are many different ways to measure arterial blood pressure. These include invasive (direct) and noninvasive (indirect) methods as well as continual and intermittent methods.
Intra-arterial catheterization is a very invasive and direct method of arterial blood pressure measurement. Intra-arterial catheterization, however, is the most reliable and accurate method for blood pressure measurement because it provides direct and continuous monitoring of the pressure actually in a major artery such as the descending aorta. The risk, however, of complications such as thrombosis, embolism, and infections limits the use of intra-arterial catheterization for blood pressure monitoring to those situations where the benefits of its use outweigh the risk. Such situations include, for example, monitoring of critically ill patients in an intensive care unit or during surgical procedures in an operating room. Because of the disadvantages of direct invasive blood pressure monitoring techniques, many noninvasive techniques for the monitoring of arterial blood pressure have been proposed in the art.
In 1834, the first indirect measurement of human blood pressure was made by Herisson. Herisson's technique used a sphygmomanometer that applied a counterpressure (referred to as Pc) to the radial artery of the arm. Herisson determined a subject's systolic pressure (referred to as Ps) by noting the amount of Pc required to eliminate arterial pulsation. Still Herisson's method had several drawbacks which resulted in inaccurate measurements and the method could only determine Ps.
Later, Riva-Rocci introduced an air-inflated arm-occluding cuff which improved the determination of Pc. Still later, Korotkoff proposed the auscultatory method of indirect blood pressure measurement which provided for the determination of diastolic pressure (referred to as Pd) in addition to determining Ps.
In the commonly practiced modern auscultatory method, the pressure of the air inflated cuff around the subject's arm (Pc) is first raised quickly until the occlusion of the brachial artery occurs. A stethoscope placed over a brachial artery below the cuff is used to listen to the Korotkoff sounds (the sounds made as blood passes through a partially occluded artery) as the Pc is gradually released. Ps is taken to be equal to the Pc corresponding to the occurrence of the first Korotkoff sound; Pd is taken to be equal to the Pc which corresponding to the fourth or fifth Korotkoff sound.
The accuracy of the auscultatory method is limited by the arbitrary nature of the relationship between the Korotkoff sounds and the corresponding pressure values as well as the skill of the person carrying out the method. In a subject having normal blood pressure, Ps values determined by the auscultatory method tend to be low (about 3-4 mmHg) with a standard deviation (SD) of 8 mmhg, while Pd values tend to be high (about 8 mmHg) with a SD of 8 mmHg. Other recognized major disadvantages of the auscultatory method is that its accuracy degrades severely with hypotension and obesity. It is also unreliable in infants and children.
Another method of making noninvasive blood pressure measurements is the oscillometric method. The oscillometric method of blood pressure determination attempts to overcome the drawbacks which accompany the use of the auscultatory method. The oscillometric principle states that there is a Pc which causes maximum oscillation of the Pc. Furthermore, the oscillometric principle states that when the maximum oscillation of Pc is obtained, Pc is equal to the subject's mean arterial pressure (referred to as Pm).
The correspondence between the maximum oscillation of Pc and Pm, however, has been shown to be dependent on pulse pressure (Ps-Pd) with the deviation of measured Pm from actual Pm increasing with increasing pulse pressure. Even though the oscillometric principle has been recognized for almost a century, the development of the oscillometric method for measuring Ps and Pd has occurred much more recently.
From studies published in 1982, comparing the auscultatory method and the oscillometric method in human subjects, it was found that Ps was equal to the Pc corresponding to oscillation amplitudes in the range of 0.45 to 0.57 of maximum oscillations (corresponding to Pm); Pd was found to equal the Pc corresponding to the oscillation amplitudes in the range of 0.75 to 0.86 of maximum oscillations. In a study using dogs, in which the direct arterial catheterization method was used as a reference, it was found that the systolic and diastolic ratios varied from about 0.43 to 0.73 and from about 0.69 to 0.83 of maximum oscillation, respectively.
It has hitherto been seldom recognized that the measurement of Pm using the oscillometric method is likely to be inaccurate, and increasingly so in the case of young, obese, hyper- or hypo-tensive, or other patients, which are abnormal in some respect. In the previously used oscillometric method, Pd and Ps are assumed to be some fixed ratio of the changes in arterial volume which are detected by some apparatus. Upon close examination of actual patients, as explained further below, not only is the determination of Pm often inaccurate, but the assumption that Pd and Ps are a fixed ratio does not hold in cases of abnormal blood pressure waveforms or other abnormal conditions.
Other methods for noninvasive continuous arterial blood pressure measurement, such as the volume clamp method, also present their own disadvantages and difficulties. In recent years, devices which are designed to automatically and noninvasively measure blood pressure have been introduced. Both the volume clamp and the oscillometric methods of blood pressure measurement have been used in mechanical, automated devices to noninvasively provide blood pressure determinations.
One of the automated blood pressure measuring devices which has been introduced, and which uses the oscillometric principle, is marketing under the trademark DINAMAP.TM. and available from Critikon. Another automated blood pressure measuring device which is now available is marketed under the trademark FINAPRES.TM.. The FINAPRES.TM. device uses the volume clamp method of blood pressure determination.
Both the DINAMAP.TM. device and the FINAPRES.TM. device have serious drawbacks and disadvantages, most of which are inherent in the method of blood pressure determination used thereby. For example, it has been determined that these noninvasive blood pressure measuring techniques produce results which significantly differ from measurements obtained by direct catheterization in anesthetized surgical patients. In hypertensive patients, systematic errors in Ps as large as -53.3 mmHg have been reported. In hypotensive patients, systematic errors in Pd as large as 16.6 mmHg have been reported. It has been reported that the DINAMAP.TM. device routinely underestimates Ps in hypertensive patients and overestimates Pd in hypotensive patients.
In an effort to overcome the problems associated with frequent inflation of arm cuffs, devices applying pressure to a finger (i.e., a digital artery) have been developed. Alternative schemes using the oscillometric principal have been developed which permit finger site application with results comparable to those of the oscillometric method applied to other sites on the body. The finger site oscillometric method uses photoelectric plethysmography to provide a volume oscillogram (a series of pulse volumes as a function of Pc).
Critically, the oscillometric method relies upon assumptions and empirical data which may not be accurate under the most important circumstances, i.e., in the hypotensive or hypertensive patient. These assumptions include the assumption that Pd and Ps are always a fixed ratio of the detected changes in arterial volume in all patients; an assumption which heretofore has not been recognized as false in many important circumstances. Thus, it would be a great advance in the art to provide a noninvasive blood pressure measuring system and method which produces accurate measurements for both normal and abnormal subjects.